Orthodontic treatment involves movement of malpositioned teeth to orthodontically correct positions. Tiny orthodontic appliances known as brackets are connected to exterior surfaces of the patient's teeth, and an archwire is placed in a slot of each bracket. The archwire forms a track to guide movement of the teeth to desired positions for correct occlusion. End sections of the archwire are often received in appliances known as buccal tubes that are fixed to the patient's molar teeth. In recent years it has become common practice to bond orthodontic appliances directly to the surface of the tooth.
For many years, it was common practice to apply orthodontic adhesive to the base of directly-bonded appliances immediately before the appliances were placed on the tooth. In some instances, a quantity of adhesive was dispensed onto a mixing pad or dispensing well and a small spatula or other hand instrument was then used to apply a small dab of adhesive to each appliance. In other instances, a quantity of adhesive was dispensed from a syringe directly onto the base of the appliance.
Adhesive precoated brackets are known and offer significant advantages to the orthodontist. Adhesive precoated brackets have a bonding base upon which the manufacturer may apply a precise quantity of adhesive such as a photocurable adhesive. When it is desired to mount the bracket on a tooth, the bracket is simply removed from the package and placed directly onto the tooth surface.
Photocurable adhesives offer advantages to orthodontic practitioners. In many types of orthodontic treatment, the exact position of the bracket on the corresponding tooth is highly important in order to facilitate moving the teeth to the desired positions without the need to place bends or twists in the archwire. As a consequence, the use of a photocurable orthodontic adhesive is highly beneficial in that the practitioner can take time to position the brackets in precise, appropriate locations. Once the practitioner is satisfied with the bracket positions, the curing light can be activated to quickly harden the adhesive and secure the brackets in place.
In addition to good handling properties and adequate adhesion, practitioners often desire that photocurable adhesives provide other desirable functional properties, including, for example, sustained fluoride release. In many applications, it is also desirable that the adhesive have adequate hydrophilicity or moisture tolerance, as indicated, for example, by the ability of the adhesive to absorb water or saliva.
Some attempts at precoating brackets with adhesives used adhesives that were more viscous (i.e., less fluid) than other available orthodontic adhesives. Higher viscosity was used to ensure that the adhesive retained its shape and did not separate or distort when the bracket was lifted from the package for use. However, some orthodontists prefer the use of less viscous (i.e., more fluid) adhesives in order to facilitate manipulation of the bracket before the adhesive is cured. For example, brackets with less viscous adhesives are relatively easy to slide along the tooth surface when an effort is made to align the bracket in a proper, precise orientation on the tooth before the adhesive is cured. However, the use of an adhesive with too low of a viscosity may be detrimental by causing the bracket to slip in the package or slip on the tooth (i.e., skating).
Orthodontic brackets precoated with adhesive are commonly sold with a flexible release substrate in contact with the adhesive. However, many release substrates are not suitable for use with all adhesives. For example, when soft, tacky, less viscous, hydrophilic adhesives are used, it has been found that a portion of the adhesive is sometimes left on the release substrate when an attempt is made to lift the appliance from the container and detach the adhesive from the release substrate. In those instances, sufficient adhesive may not remain on the bracket to provide adequate bond strength to retain the bracket on the tooth during the course of orthodontic treatment. Moreover, adhesives having a relatively low viscosity tend to slowly flow across the release substrate and from the space beneath the bracket over extended periods of time, creating bracket removal and/or liner release problems.
Additionally, when soft, tacky, less viscous adhesives are used with conventional adhesive precoated appliance packages, the shape of the adhesive may distort as the appliance is lifted from the container. In some instances, detaching the adhesive from the release substrate may alter the configuration of the adhesive, resulting in unsatisfactory direct bonding unless additional steps are undertaken to shift the adhesive by hand back to its original, pillow-like shape. For example, when lifting an appliance from the container, some of the adhesive may be shifted to one side of the appliance base, such that the opposite side of the base does not have a satisfactory amount of adhesive for bonding. If the appliance is secured to the tooth in such a manner that a void space is present between a portion of the base and the opposed tooth surface, the void space may result in premature, spontaneous debonding of the appliance from the tooth, a nuisance that is best avoided. Furthermore, in some instances the void space can establish a pocket that receives food and debris, facilitating the formation of caries.
In addition, there has been an increased interest in the use of adhesives that change from a noticeable color to a lack of color when curing. When uncured, such adhesives are easily observed, thus facilitating the clean-up of excess adhesive after the appliance has been positioned on the tooth surface. Once the adhesive has cured, the color is substantially eliminated, rendering the adhesive more difficult to see and, therefore, more aesthetic during the course of treatment.
Presently, there is a need in the art for precoated orthodontic appliances, wherein the adhesive provides a balance of properties and characteristics that are desired by the practitioner.